Silane coating of metal substrates: Complementary use of electrochemical, optical and thermal analysis for the evaluation of film properties

Silane coating is a metal surface pre-treatment introduced as a replacement for chromium treatments. It is supposed to be suited for various metals, including aluminium, steel and galvanised steel. Good corrosion performance has been assigned to these hybrid organic–inorganic thin film deposition systems. The silane molecules contain SiOC n H(2n+1) groups, which after hydrolysis into reactive silanol groups SiOH form a covalent bonded layer on the metal surface. Curing of these films is considered essential for corrosion protection; during heat treatment, condensation of unreacted silanol groups in the film result in the formation of a SiOSi network with enhanced barrier properties. This contribution gives a chemical and morphological characterisation, obtained with spectroscopic ellipsometry (SE), of BTSE silane coated aluminium as a function of curing. Further, with thermal gravimetric analysis combined with mass spectroscopy (TGA-MS), the mechanism and kinetics of the curing process are explained, and the resulting barrier properties are measured using electrochemical impedance spectroscopy (EIS).